The bagworm, Thyridopteryx ephemeraeformis, is a destructive pest that attacks both evergreen and deciduous plants. Uncontrolled feeding can lead to the complete defoliation of trees, especially conifers like juniper and arborvitae, potentially resulting in plant death. Effective control relies on precise timing, as the bagworm is only vulnerable to insecticides for a brief period each year.
Understanding the Bagworm Life Cycle
Bagworms spend the winter as eggs inside the tough, spindle-shaped bags constructed by the previous generation’s female. These bags, which can hold 500 to 1,000 eggs, remain suspended from twigs and branches. The eggs typically begin to hatch in late spring, usually between late May and early June, depending on local climate conditions.
The newly emerged larvae are tiny, about two millimeters long, and immediately begin spinning a small silken bag around themselves, incorporating foliage for camouflage. This early larval stage, when the bags are small and the caterpillars are actively feeding, is the only window when chemical or biological treatments are effective. As the season progresses, the larvae mature, the bags become larger and tougher, and the caterpillars are less exposed. By late summer or fall, treatments become virtually ineffective. By August, the larvae stop feeding, permanently attach their bags to a branch, and prepare to pupate, closing the window for intervention.
Targeting Bagworms with Synthetic Insecticides
For heavy infestations, synthetic insecticides provide a strong defense, acting as contact killers that offer residual protection on the foliage. These products are most effective against young larvae, typically when the bags are less than an inch long. Common active ingredients include pyrethroids, such as permethrin, bifenthrin, and cyfluthrin.
Pyrethroids are broad-spectrum insecticides that kill the larvae upon contact or ingestion and remain active on plant surfaces for an extended period. Carbaryl, often sold as Sevin, is another traditional option that acts as a contact and stomach poison for feeding caterpillars. Thorough coverage of the plant’s foliage is necessary for the larvae to encounter the residue while feeding.
Some professionals use systemic insecticides, such as acephate, which are absorbed by the plant and move through its vascular system. While this offers application flexibility, the insecticide must be absorbed before the larvae begin to feed, making spring timing important. Because these synthetic options are broad-spectrum, they can pose a threat to non-target insects, including beneficial species and pollinators. They should be used judiciously and according to label instructions.
Biological and Non-Chemical Alternatives
The biological insecticide Bacillus thuringiensis subspecies kurstaki (Bt) is the primary alternative to synthetic chemicals for bagworm control. Bt is a naturally occurring bacterium that must be ingested by the caterpillar to be effective. Once consumed, the bacterium releases a toxin that paralyzes the bagworm’s digestive system, causing it to stop feeding and die within a few days.
Bt is highly selective, meaning it only affects caterpillars and is considered safe for beneficial insects, fish, wildlife, and people. This treatment is dependent on timing and larval size; it must be applied when the larvae are very young and actively feeding, usually when bags are less than three-quarters of an inch long. If the bagworms are larger or have already sealed their bags, Bt will not be effective.
Manual Removal
The most direct non-chemical method is manual removal, or handpicking, of the bags during late fall, winter, or early spring. Removing the bags before May eliminates the overwintering eggs and prevents the next generation of larvae from emerging. Once removed, the bags should be destroyed or placed in a sealed container. Simply throwing them on the ground allows the eggs to hatch and the larvae to crawl onto the nearest host plant.
Achieving Maximum Effectiveness Through Proper Application
The success of any bagworm treatment hinges on precisely confirming the local hatch timing. Since the egg hatch can vary by several weeks depending on regional weather, contact a local cooperative extension office for exact dates. Alternatively, monitor for indicator plants, such as the blooming of catalpa trees or Japanese tree lilacs. Applying any insecticide too early or too late results in wasted effort and continued plant damage.
Once the young larvae are present, the application must ensure thorough coverage of the entire plant, as the caterpillars must either contact the chemical or ingest the treated foliage. This includes soaking the tops and undersides of leaves or needles, since small bagworms are easily missed and can hide. Using appropriate personal protective equipment (PPE) and following the product label instructions exactly is mandatory, as the label is the legal guide for safe and effective use.
A follow-up inspection about ten days after the initial application is important to assess the control achieved. If the infestation was severe or if heavy rain followed the application, a reapplication may be necessary, particularly with biological controls like Bt, which degrade quickly in sunlight. Monitoring is an ongoing process, as new larvae can be blown in from nearby infested trees by the wind, an event called “ballooning,” necessitating continued vigilance.